Method of calibrating a patient monitoring system for use with a radiotherapy treatment apparatus

ABSTRACT

A method of calibrating a monitoring system (10,14) is described in which a calibration phantom (70) is located with its center located approximately at the isocenter of a treatment room through which a treatment apparatus (16) is arranged to direct radiation, wherein the surface of the calibration phantom (70) closest to an image capture device (72) of the monitoring system (10,14) is inclined approximately 45° relative to the camera plane of an image capture device of the monitoring system. Images of the calibration phantom (70) are then captured using the image capture device (72) and the images are processed to generate a model of the imaged surface of the calibration phantom. The generated model of the imaged surface of the calibration phantom (70) is then utilized to identify the relative location of the center of the calibration phantom (70) and the camera plane of the image capture device (72) which is then utilized to determine the relative location of the camera plane of the image capture device and the isocenter of a treatment room.

This application is a Continuation of co-pending application Ser. No.16/635,884, filed on Jan. 31, 2020, which is a National Phase of PCTInternational Application No. PCT/GB2018/052192 filed on Jul. 31, 2018,which claims priority under 35 U.S.C. § 119(a) to UK Patent ApplicationNo. GB 1712462.9 filed on Aug. 2, 2017. Each of the above applicationsis hereby expressly incorporated by reference, in its entirety, into thepresent application.

The present invention concerns a method of calibrating a patientmonitoring system for monitoring the location of a patient duringradiotherapy. In particular, the present invention concerns a method ofidentifying the locations of image detectors such as cameras in amonitoring system relative to a treatment room isocenter.

Radiotherapy consists of projecting onto a predetermined region of apatient's body, a radiation beam so as to destroy or eliminate tumorsexisting therein. Such treatment is usually carried out periodically andrepeatedly. At each medical intervention, the radiation source must bepositioned with respect to the patient in order to irradiate theselected region with the highest possible accuracy to avoid radiatingadjacent tissue on which radiation beams would be harmful. For thisreason, a number of monitoring systems for assisting the positioning ofpatients during radiotherapy have been proposed such as those describedin Vision RT's earlier patents and patent applications in U.S. Pat. Nos.7,889,906, 7,348,974, 8,135,201, 9,028,422, and US Pat Application Nos.2015/265852 and 2016/129283, all of which are hereby incorporated byreference.

In the systems described in Vision RT's patents and patent applications,images of a patient are obtained and processed to generate dataidentifying 3D positions of a large number of points corresponding topoints on the surface of a patient. Such data can be compared with datagenerated on a previous occasion and used to position a patient in aconsistent manner or provide a warning when a patient moves out ofposition. Typically such a comparison involves undertaking Procrustesanalysis to determine a transformation which minimizes the differencesin position between points on the surface of a patient identified bydata generated based on live images and points on the surface of apatient identified by data generated on a previous occasion.

Vision RT's patient monitoring systems are able to generate highlyaccurate (e.g. sub millimeter) models of the surface of a patient. To doso, the monitoring system is calibrated in order to establish cameraparameters identifying the relative locations and orientations of theimage capture devices/cameras, any optical distortion caused by theoptical design of the lens of each image detector/camera e.g. barrel,pincushion, and moustache distortion and de-centering/tangentialdistortion, and other internal parameters of the cameras/image capturedevices (e.g. focal length, image center, aspect ratio skew, pixelspacing etc.). Once known, camera parameters can be utilized tomanipulate obtained images to obtain images free of distortion. 3Dposition measurements can then be determined by processing imagesobtained from different locations and deriving 3D positions from theimages and the relative locations and orientations of the image capturedevices/cameras.

In addition, the monitoring system must also be calibrated so as toidentify the relative location of the cameras of the monitoring systemand the treatment room isocenter towards which radiation generated by atreatment apparatus is directed.

Originally, the primary method for isocenter verification inradiotherapy was to measure the distance between the tip of a mechanicalpointer mounted on the gantry head of a treatment apparatus and a fixedpoint mounted on the treatment table. Such a method was manual,laborious and time-consuming. The accuracy of the method depended uponthe human observer and was also limited by size of the tip of thepointer used.

An improved technique was introduced by Lutz, Winston and Maleki atHarvard Medical School in 1988 which is described in Lutz W, Winston KR, Maleki N. A system for stereotactic radiosurgery with a linearaccelerator. Int J Radiat Oncol Biol Phys. 1988; 14(2):373-81. In theWinston-Lutz system, a calibration phantom comprising a small metallicball made of steel, titanium or tungsten is fixed on the treatment tableby a locking mechanism. The phantom position is adjustable in threedirections by means of a micrometer tool. The collimator used forradiotherapy is attached to the gantry head and the ball is placed asclosely as possible to the isocenter by aligning the marks on thephantom with the treatment room lasers. The collimated beam is used toexpose a radiographic test film mounted perpendicular to the beamdirection on a stand behind the ball. Differences between the center ofthe sphere shadow and the field center identifies the differencesbetween the true isocenter and the isocenter as indicated by thetreatment room lasers. The offset is read on each film using transparenttemplate guidance scales or by scanning the film and software analysis.

A mathematical method for analyzing radiographic Winston Lutz images wasdeveloped and is described in Low D A, Li Z, Drzymala R E. Minimizationof target positioning error in accelerator-based radiosurgery. Med Phys.1995; 22(4):443-48 which used the film-measured isocenter positionalerrors for eight gantry angle and couch settings to find the suitableoffset for the phantom stand to minimize the distance between thetreatment apparatus isocenter and the target. A similar aim was followedby Grimm et al., who developed an algorithm to reconstruct theWinston-Lutz phantom ball locus in three dimensions from two-dimensionalradiographic film images taken at certain couch and gantry angles andcombined them with the images of lasers taken by digital cameras. Thisapproach is described in Grimm J, Grimm S L, Das I J, et al. A qualityassurance method with sub-millimeter accuracy for stereotactic linearaccelerators. J Appl Clin Med Phys. 2011; 12(1):182-98.

A further example of automated processing of radiographic phantom imagesis described in E Schriebmann, E Elder and T Fox, Automated QualityAssurance for Image-Guided Radiation Therapy, J Appl Clin Med Phys.2009: 10(1):71-79 which discusses the automation of Quality Assurancemethods to ensure that a megavoltage (MV) treatment beam coincides withan integrated kilo voltage (kV) or volumetric cone beam CT. In thepaper, a calibration cube is described as being located at the estimatedlocation of treatment room isocenter using laser markings. Radiographicimages of the irradiation of the cube are then obtained and processed todetermine the extent the cube as positioned is offset from the isocenteras identified by the MV, kV and volumetric cone beams.

The position of the calibration phantom can then be adjusted based onthe analysis of the radiographic Winston Lutz images until the phantomis accurately located at the treatment room isocenter. Having identifiedthe location of the isocenter, the location of the isocenter can then behighlighted by using a set of lasers generating planes of laser lightsand to that end many calibration phantoms have exterior markings so thatonce the phantom has been located at the isocenter, laser lights can beadjusted so that generated planes of laser light coincide with theexterior markings and when the phantom is removed, the location of theisocenter is identified by the intersection of the laser beams.

The positioning of the cameras relative to the isocenter of thetreatment apparatus can then be determined by imaging a calibration cubeof known size which is positioned on a treatment apparatus at a positionwith its center at the isocenter of the treatment apparatus. Typicallypositioning the calibration cube is achieved through the co-incidence ofmarks on the exterior of the cube with the projection of the laser crosshairs which intersect at the isocenter. Images of the calibration cubeare then obtained and processed utilizing the previously obtainedmeasurements of the relative locations of the cameras and any data aboutthe existence of any distortion present in the images and a 3D computermodel of the surface of the cube is generated. A comparison between thegenerated 3D model and the known parameters for the size and position ofthe calibration cube enables measurements made in the co-ordinate systemof the modelling software to be converted into real world measurementsin the treatment room relative to the treatment isocenter.

Although the conventional approach to calibrating a stereoscopic camerasystem for use with a radio therapy treatment apparatus is highlyaccurate, further improvements in accuracy are desirable.

In accordance with one aspect of the present invention a method ofcalibrating a monitoring system is provided characterized by acalibration phantom being located with its center located approximatelyat the isocenter of a treatment room wherein in plan-view the surface ofthe calibration phantom closest to an image capture device of themonitoring system is inclined approximately 45° relative to the cameraplane of an image capture device of the monitoring system. Images of thecalibration phantom are then captured using the image capture device andthe images are processed to generate a model of the imaged surface ofthe calibration phantom. The generated model of the imaged surface ofthe calibration phantom is then utilized to identify the relativelocation of the center of the calibration phantom and the camera planeof the image capture device which in turn is utilized to determine therelative location of the camera plane of the image capture device andthe isocenter of a treatment room through which a treatment apparatus isarranged to direct radiation.

The applicants have determined that angling the surface of a calibrationphantom, in particular a calibration cube, so that in plan-view thesurface of the calibration phantom closest to an image capture device isinclined approximately 45° relative to the camera plane of an imagecapture device of a monitoring system improves the accuracy with whichsuch a monitoring system can generate a model of the calibration phantomand hence improves the accuracy with which the relative locations ofimage capture devices and a treatment room isocenter may be determined.

In some embodiments, locating a calibration phantom with its centerlocated approximately at the isocenter of a treatment room, wherein inplan-view the surface of the calibration phantom closest to an imagecapture device is inclined approximately 45° relative to the cameraplane of the image capture device may comprise using a laser lightingsystem to highlight the isocenter of a treatment room and positioningthe a calibration phantom by aligning laser light used to highlight theisocenter of a treatment room with markings provided on the exterior ofthe calibration phantom. The markings on a calibration phantom in theform of a calibration cube may comprise markings extending along theedges of the cube; markings bisecting the cube; and/or a cross extendingbetween diagonally opposite corners of the cube.

In some embodiments the calibration phantom may comprise a calibrationphantom containing an irradiation target. In such embodiments locatingthe relative positions of an image capture device of a monitoring systemfor monitoring the positioning of a patient during radiation treatmentand an isocenter of a treatment room through which a treatment apparatusis arranged to direct radiation may comprise obtaining a radiographicimage of the calibration phantom irradiated by the treatment apparatusand analyzing the obtained radiographic image to determine the relativelocation of the treatment room isocenter and the center of thecalibration phantom.

In such embodiments the calibration phantom may be repositioned afterthe relative location of the treatment room of isocenter and the centerof the calibration phantom has been identified and images of therepositioned phantom may be obtained by the monitoring system andutilized to determine the relative location of an image capture deviceand an isocenter of a treatment room.

Alternatively, the relative location of the treatment room of isocenterand the center of the calibration phantom may be used together with agenerated model of the calibration phantom without relocating thephantom to determine the relative location of an image capture deviceand an isocenter of a treatment room.

In some embodiments the monitoring system may comprise a plurality ofimage capture devices.

In some embodiments the monitoring system may comprise a projectoroperable to project light onto the surface of an object located in thevicinity of a treatment room isocenter. Such a projector may comprise aprojector operable to project structured light in the form of a gridpattern or a line of laser light onto the surface of an object locatedin the vicinity of the treatment room isocenter. In such a systemobtained images of the calibration phantom onto which structured lighthas been projected may be processed to analyze the distortion of aprojected pattern of structured light appearing in the images in orderto generate a model of the imaged surface of the calibration phantom.

In other embodiments the projector may comprise a projector operable toproject a speckled pattern of light onto the surface of an objectlocated in the vicinity of the treatment room isocenter. In such asystem, the monitoring system may comprise a stereoscopic camera systemand processing an obtained image to generate a model of the imagedsurface of the calibration phantom may comprise processing obtainedstereoscopic images of the calibration phantom onto which a speckledpattern of light has been projected and generating a model of the imagedsurface of the calibration phantom to identify corresponding portions ofthe object in the obtained images.

The monitoring system may comprise one or more camera pods eachcontaining one or more image capture devices wherein the camera pods aresuspended from the ceiling of the treatment room. Where the monitoringsystem comprises a plurality of camera pods, the camera pods may all belocated on the same side of a treatment room and the camera pods may bearranged in a symmetrical pattern within the treatment room.

In such embodiments a calibration phantom may be arranged with the itscenter located approximately at the isocenter of a treatment room, wherein plan-view the surface of the calibration phantom closest to an imagecapture device of one of the camera pods is inclined approximately 45°relative to the camera plane of the image capture device on that camerapod.

In some such embodiments the monitoring system may comprise a centralcamera pod flanked by two other camera pods and locating the calibrationphantom may comprise locating a calibration phantom approximately at atreatment room isocenter with the surface of the calibration phantomclosest to an image capture device of the centrally located camera podin plan-view inclined approximately 45° relative to the camera plane ofthe image capture device on the centrally located camera pod.

In a further aspect of the present invention there is provided acalibration phantom for use in any of the above described methods. Sucha calibration phantom may comprise a calibration cube bearing markingson its exterior facilitating the orientation of the cube for imaging bythe camera(s)/image capture device(s) of a monitoring system. Suchmarkings may comprise markings selected from markings extending alongthe edges of the cube; markings bisecting the cube; and a crossextending between diagonally opposite corners of the cube. In someembodiments the calibration phantom may contain an irradiation targetoperable to be irradiated by a treatment apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described in greaterdetail with reference to the accompanying drawings in which:

FIG. 1 is a schematic perspective view of a treatment apparatus and apatient monitor;

FIG. 2 is a front perspective view of a camera pod of the patientmonitor of FIG. 1;

FIG. 3 is a schematic block diagram of the computer system of thepatient monitor of FIG. 1;

FIG. 4 is a plan view of a conventional arrangement of a calibrationcube for identifying the relative location of a camera system and theisocenter of a treatment room;

FIG. 5 is a schematic perspective view of a conventional calibrationcube for identifying the relative location of a camera system and theisocenter of a treatment room;

FIG. 6 is a plan view of an arrangement of a calibration cube foridentifying the relative location of a camera system and the isocenterof a treatment room in accordance with an embodiment of the presentinvention;

FIG. 7 is a schematic perspective view of a calibration cube foridentifying the relative location of a camera system and the isocenterof a treatment room in accordance with an embodiment of the presentinvention; and

FIGS. 8-10 are plan views of arrangements of a calibration cube foridentifying the relative location of a camera system and the isocenterof a treatment room in accordance with further embodiments of thepresent invention.

SPECIFIC EMBODIMENTS

Prior to describing a method of determining the relative location ofcameras/image detectors of a monitoring system for monitoring thepositioning of a patient and an isocenter of a treatment room inaccordance with the present invention, a patient monitoring system andradiotherapy treatment apparatus which can be calibrated using thedescribed method and a conventional approach to identifying the relativelocations of cameras/image detectors of such a system and the isocenterof a treatment room will first be described with reference to FIGS. 1-5.

FIG. 1 is a schematic perspective view of an exemplary patientmonitoring system comprising a camera system comprising a number ofcameras mounted within a number of camera pods 10 one of which is shownin FIG. 1 that are connected by wiring (not shown) to a computer 14. Thecomputer 14 is also connected to treatment apparatus 16 such as a linearaccelerator for applying radiotherapy. A mechanical couch 18 is providedas part of the treatment apparatus upon which a patient 20 lies duringtreatment. The treatment apparatus 16 and the mechanical couch 18 arearranged such that, under the control of the computer 14, the relativepositions of the mechanical couch 18 and the treatment apparatus 16 maybe varied, laterally, vertically, longitudinally and rotationally as isindicated in the figure by the arrows adjacent the couch.

The treatment apparatus 16 comprises a main body 22 from which extends agantry 24. A collimator 26 is provided at the end of the gantry 24remote from the main body 22 of the treatment apparatus 16. To vary theangles at which radiation irradiates a patient 20, the gantry 24, underthe control of the computer 14, is arranged to rotate about an axispassing through the center of the main body 22 of the treatmentapparatus 16 as indicated on the figure. Additionally the direction ofirradiation by the treatment apparatus may also be varied by rotatingthe collimator 26 at the end of the gantry 24 as also indicated by thearrows on the figure.

To obtain a reasonable field of view in a patient monitoring system,cameras pods 10 containing cameras monitoring a patient 20, typicallyview a patient 20 from a distance (e.g. 1 to 2 meters from the patientbeing monitored). In the exemplary illustration of FIG. 1, the field ofview of the camera pod 10 shown in FIG. 1 is indicated by the dashedlines extending away from the camera pod 10.

As is shown in FIG. 1, typically such camera pods 10 are suspended fromthe ceiling of a treatment room and are located away from the gantry 24so that the camera pods 10 do not interfere with the rotation of thegantry 24. In some systems a camera system including only a singlecamera pod 10 is utilized. However, in other systems, it is preferablefor the camera system to include multiple camera pods 10 as rotation ofthe gantry 24 may block the view of a patient 20 in whole or in partwhen the gantry 24 or the mechanical couch 18 are in particularorientations. The provision of multiple camera pods 10 also facilitatesimaging a patient from multiple directions which may increase theaccuracy of the system.

A laser lighting system (not shown), typically in the form of a set oflaser lights arranged to generate three planes of laser light may beprovided to highlight the treatment room isocenter, being the positionin the treatment room, through which the treatment apparatus 16 isarranged to direct radiation regardless of the orientation and positionof the collimator 26 and gantry 24. When a patient 20 is positioned fortreatment, this treatment room isocenter should coincide with the tissueintended to receive the greatest amount of radiation,

FIG. 2 is a front perspective view of an exemplary camera pod 10.

The camera pod 10 in this example comprises a housing 41 which isconnected to a bracket 42 via a hinge 44. The bracket 42 enables thecamera pod 10 to be attached in a fixed location to the ceiling of atreatment room whilst the hinge 44 permits the orientation of the camerapod 10 to be orientated relative to the bracket 42 so that the camerapod 10 can be arranged to view a patient 20 on a mechanical couch 18. Apair of lenses 46 are mounted at either end of the front surface 48 ofthe housing 41. These lenses 46 are positioned in front of image capturedevices/cameras such as CMOS active pixel sensors or charge coupleddevices (not shown) contained within the housing 41. The cameras/imagedetectors are arranged behind the lenses 46 so as to capture images of apatient 20 via the lenses 46.

In this example, a speckle projector 52 is provided in the middle of thefront surface 48 of the housing 41 between the two lenses 46 in thecamera pod 10 shown in FIG. 2. The speckle projector 52 in this exampleis arranged to illuminate a patient 20 with a non-repeating speckledpattern of red light so that when images of a patient 20 are captured bythe two image detectors mounted within a camera pod 10 correspondingportions of captured images can be more easily distinguished. To thatend the speckle projector comprises a light source such as a LED and afilm with a random speckle pattern printed on the film. In use lightfrom the light source is projected via the film and as a result apattern consisting of light and dark areas is projected onto the surfaceof a patient 20. In some monitoring systems, the speckle projector 52could be replaced with a projector arranged to project structured light(e.g. laser light) in the form of a line or a grid pattern onto thesurface of a patient 20. In some monitoring systems, the projector 52could be omitted.

FIG. 3 is a schematic block diagram of the computer 14 of the patientmonitor of FIG. 1. In order for the computer 14 to process imagesreceived from the camera pods 10, the computer 14 is configured bysoftware either provided on a disk 54 or by receiving an electricalsignal 55 via a communications network into a number of functionalmodules 56-64. In this example, the functional modules 56-64 comprise: a3D position determination module 56 for processing images received fromthe stereoscopic camera system 10, a model generation module 58 forprocessing data generated by the 3D position determination module 56 andconverting the data into a 3D wire mesh model of an imaged surface; agenerated model store 60 for storing a 3D wire mesh model of an imagedsurface; a target model store 62 for storing a previously generated 3Dwire mesh model; and a matching module 64 for determining rotations andtranslations required to match a generated model with a target model.

In use, as images are obtained by the image capture devices/cameras ofthe camera pods 10, these images are processed by the 3D positiondetermination module 56. This processing enables the 3D positiondetermination module to identify 3D positions of corresponding points inpairs of images on the surface of a patient 20. In the exemplary system,this is achieved by the 3D position determination module 56 identifyingcorresponding points in pairs of images obtained by the camera pods 10and then determining 3D positions for those points based on the relativepositions of corresponding points in obtained pairs of images and storedcamera parameters for each of the image capture devices/cameras of thecamera pods 10.

The position data generated by the 3D position determination module 56is then passed to the model generation module 58 which processes theposition data to generate a 3D wire mesh model of the surface of apatient 20 imaged by the stereoscopic cameras 10. The 3D model comprisesa triangulated wire mesh model where the vertices of the modelcorrespond to the 3D positions determined by the 3D positiondetermination module 56. When such a model has been determined it isstored in the generated model store 60.

When a wire mesh model of the surface of a patient 20 has been stored,the matching module 64 is then invoked to determine a matchingtranslation and rotation between the generated model based on thecurrent images being obtained by the stereoscopic cameras 10 and apreviously generated model surface of the patient stored in the targetmodel store 62. The determined translation and rotation can then be sentas instructions to the mechanical couch 18 to cause the couch toposition the patient 20 in the same position relative to the treatmentapparatus 16 as the patient 20 was were when the patient 20 waspreviously treated.

Subsequently, the image capture devices/cameras of the camera pods 10can continue to monitor the patient 20 and any variation in position canbe identified by generating further model surfaces and comparing thosegenerated surfaces with the target model stored in the target modelstore 62. If it is determined that a patient 20 has moved out ofposition, the treatment apparatus 16 can be halted or a warning can betriggered and the patient 20 repositioned, thereby avoiding irradiatingthe wrong parts of the patient 20.

A conventional approach to identifying the relative location of thecameras of a monitoring system relative to the isocenter of a treatmentroom will now be described with reference to FIGS. 4 and 5.

FIG. 4 is a schematic plan view of calibration phantom which in thisexample is in the form of a calibration cube 70 being imaged by threecameras or camera pods 72-76. When identifying the relative location ofthe cameras of a monitoring system relative to the isocenter of atreatment room, the calibration cube 70 is located with the center ofthe calibration cube 70 at the isocenter of the treatment room. This isachieved by identifying the location of the isocenter using conventionaltechniques and highlighting the location of the isocenter by theintersection of three planes of laser light, two of which 80, 82 areshown by the heavy dashed lines in FIG. 4. The third plane (notillustrated in FIG. 4) would be orientated parallel with the surface ofthe Figure identifying the “height” of the isocenter relative to thesurface of the Figure.

As is shown in the Figure, typically the camera pods 72-76 are located adistance away from the location of the isocenter so as not to interferewith the movement of the treatment apparatus 16 as it irradiates theisocenter from different angles and positions. Typically in a patientmonitoring system three camera pods 72-76 are provided and, as shown inFIG. 4, are arranged with a central camera pod 72 flanked by two othercamera pods 74,76. These secondary camera pods 74,76 are often locatedas is shown in FIG. 4 symmetrically either side of the central camerapod 72 and on the same side of the treatment room isocenter as thecentral camera pod 72. In some systems, secondary camera pods 74,76 maybe arranged so to be located on either side of the isocenter andsubstantially in line with the isocenter (i.e. with a line of sightsubstantially orientated along the line of plane of laser light 80highlighting the isocenter of the treatment room).

When identifying the relative location of the cameras of a monitoringsystem relative to the isocenter of a treatment room, the calibrationcube 70 is orientated so that in plan-view one of the surfaces of thecalibration cube 70 is substantially parallel with the image plane ofthe cameras/image detectors of the central camera pod 72. Typically thisorientation is achieved by aligning the image plane of the centralcamera pod 72 in plan-view so as to be parallel with one of the planes80 of light highlighting the position of the treatment room isocenter.Markings are then provided on the surfaces of the cube enabling thecalibration cube 70 to be correctly aligned so that the surface of thecube facing the central camera pod 72 is parallel with the image planesof the cameras/image detectors of the central pod 72 and positioned withthe center of the calibration cube 70 at the treatment room isocenter70. Typically such markings are in the form of a cross 84 on each of thesurfaces of the calibration cube, as is illustrated in FIG. 5.

When generating a model from images of a calibration cube 70, theaccuracy with which a model can be generated typically decreases forsurfaces imaged at an oblique angle and for that reason surfaces of acalibration cube 70 are typically preferentially modelled using imagedata which views a surface at the least oblique angle.

Orientating the cube, with a front surface of the calibration cube 70substantially parallel with the image plane of cameras/image detectorsof a central camera pod 72, minimizes the angle at which the centralcamera pod 72 views the calibration cube 70 and, in an orientation suchas is illustrated in FIG. 4, the other camera pods 74,76 view thesurfaces of the calibration cube 70 closest to them at a slight angle.

With the calibration cube 70 located with the center of the cube locatedat the treatment room isocenter and with the surface of the cube 70parallel with the image plane of the cameras/image detectors of thecentral camera pod 72, the planes of laser light highlighting theisocenter of the treatment room should coincide with the markings 84 onthe calibration cube 70.

When the calibration cube 70 has been located with the center of thecube located at the treatment room isocenter, the cameras/imagedetectors of the camera pods 72-76 capture images of the calibrationcube 70. These images are then passed to the computer 14 which processesthe images to identify the 3D locations of points on the surface of thecalibration cube 70. The relative location of the treatment roomisocenter in the model space of the monitoring system can then beidentified as the center of a best fit for a model of the calibrationcube 70 to the identified points of the surface of the cube based on theprocessed images.

The applicants have appreciated that where, as is typical in a patientmonitoring system, multiple camera pods are located on the same side ofa treatment room isocenter, when imaging a calibration cube 70,typically none of the cameras obtain images of the surface of thecalibration cube 70 which lies on the other side of the isocenter.

Thus for example in the case of the monitoring system illustrated inFIG. 4, where the view points of the camera pods 72-76 are illustratedby dotted lines extending away from the schematic illustrations of thecamera pods 72-76, none of the camera pods 72-76 obtain images of thesurface of the calibration cube 70 remote from the central camera pod72, highlighted in FIG. 4 as the surface of the calibration cube 70 onthe right hand side of the cube 70 and highlighted as a dotted line.

The applicants have further appreciated that this failure to obtainimages of that surface of the calibration cube 70 is a potential sourceof error in determining the location of the treatment isocenter in themodel space of the monitoring system.

To address this issue, the applicants propose that rather thanpositioning a calibration cube 70 in the manner illustrated in FIG. 4where the surface of the calibration cube 70 is aligned to besubstantially parallel with the image plane of the cameras/detectors ofa central camera pod 72, the calibration should instead be orientated asillustrated in FIG. 6 (i.e. rotated by 45° so that in plan-view thesurfaces of the calibration cube 70 closest to the central camera pod 72are at 45° relative to the image plane of the central camera pod 72 withone edge of the cube being pointed towards the camera pod 70). As shownin FIG. 6, in such an orientation the surfaces of the calibration cube70 remote from the central camera are imaged by the other camera pods74, 76.

In the orientation shown in FIG. 6, it will be appreciated that thesurfaces of the calibration cube 70 imaged by the central camera pod 72are at a more oblique angle to the image plane of the cameras/imagedetectors of the camera pod 72, than when the calibration cube 70 isorientated as in FIG. 4. Although this is the case, where the relativeangle of orientation is around 45°, the applicants have found that thisdoes not make any significant reduction in the accuracy with which the3D position of the surfaces can be modelled.

It will also be appreciated that as illustrated, the surfaces of thecalibration cube remote from the central camera pod 72 are only imagedby the other camera pods 74,76 at a relatively oblique angle. Ordinarilymodelling the 3D positions of surfaces imaged at a relatively obliqueangle is relatively inaccurate. However, although this is the case, theapplicants have determined that any inaccuracies are more thancompensated for by the ability of a monitoring system to image and hencemodel a greater proportion of the surface area of the calibration cube70.

Although not fully apparent in the plan view images of FIGS. 4 and 6, itshould be borne in mind that typically as is shown in FIG. 1 camera podsof a patient monitoring system are suspended from the ceiling of atreatment room and are located above the plane of the treatment roomisocenter. Thus in the case of the orientation of a calibration cube 70as shown in FIG. 4, a portion of the upper surface of the calibrationcube remote from the camera pods 72-76 is viewed at a more oblique anglethan portions of the surface closer to the camera pods 72-76. Thislimits the ability of the system to generate an accurate model of theportions of the cube viewed at a more oblique angel and in the case ofthe orientation illustrated in FIG. 4, typically only the portion of thecalibration cube 70 closest to the camera pods 72-76 is modelled.

Compared with the orientation illustrated in FIG. 4, additionalinformation is obtained when a calibration cube 70 orientated as shownin FIG. 6 is imaged, as even if only a small portion of the calibrationcube 70 located on the far side of the plane indicated by the plane oflaser light 80 can be modelled, the fact that three corners of the uppersurface can be identified, together with pre-knowledge of the dimensionsof the calibration cube 70, allows more accurate identification of thelocation of the calibration cube 70. Whereas in contrast, whenorientated as shown in FIG. 4, modelling the portion of the calibrationcube 70 closest to the camera pods 72-76 only results in the modellingof two of the corners of the cube.

The applicants have determined that, contrary to expectation, theadditional information about the corners of the cube, as orientated inFIG. 6, is of greater importance in identifying the location of atreatment room isocenter, than inaccuracies which arise due to thecamera pods 72-76 imaging the front portion of a calibration cube 70 ata more oblique angle or imaging the remote portions of a calibrationcube 70 only at a relatively oblique angle and hence positioning acalibration cube with surfaces in plan-view at c. 45° to the image planeof a central camera pod 72 improves the accuracy with which the atreatment room isocenter can be identified in the model space of amonitoring system.

Orientating a calibration cube 70 in the manner illustrated in FIG. 6,facilitates the modelling of the corner of the calibration cube 70closest to the central camera pod 72 using image data from all threecamera pods 72-76 both individually and collectively without relianceupon image data obtained at a highly oblique angle. This together withknowledge of the dimensions of the calibration cube 70 should enable thelocation of the center of the cube in camera space to be identified.

If all three camera pods are perfectly calibrated models of the surfacesgenerated by identifying matching portions of images from eachindividual camera pod should all be aligned. However, inevitably minorerrors do arise. In the configuration illustrated, the camera pods 72-76are arranged symmetrically. To the extent that errors arise whenmodelling the surface of the calibration cube 70 using the outer twocamera pods 74,76 such errors should cancel each other out with themodel surface generated by the central camera pod 72 identifying asurface close to the average of the surfaces generated using images fromthe other camera pods 74,76 and hence collectively the three camera podsshould enable the position of the corner of the calibration cube 70closest to the central camera pod 72 to be accurately identified withlimited error.

The positioning of a calibration cube 70 in the manner illustrated inFIG. 6, with the surface of the cube in plan-view angled at 45° relativeto the camera plane of a central camera pod 72 orientated in line withthe treatment room isocenter, can be facilitated by providing acalibration cube 70 marked as indicated in FIG. 7. In contrast to theconvention markings on a calibration cube 70 as illustrated in FIG. 5,in FIG. 7 the conventional cross markings 84 shown in FIG. 5 arereplaced by a diagonal cross 86 extending between opposite corners ofthe cube 70 on the upper surface of the cube, a line 88 bisecting thecalibration cube 70 and a series of lines 90 highlighting the edges ofthe cube 70.

When arranging the calibration cube 70 in the manner indicated in FIG.6, the modified markings 86-90 facilitate the arrangement of thecalibration cube 70 as the cube can be located with its center at thetreatment room isocenter by aligning the planes of the laser light 80,82used to identify the treatment room isocenter with the modified markings86-90 on the calibration cube 70.

Although in the above, a method of identifying the locations of camerasin a monitoring system relative to a treatment room isocenter has beendescribed in the context of a monitoring system utilizing a stereoscopiccamera system, it will be appreciated that the above described methodwhere the front surface of a calibration cube in plan-view is imaged atan angle of approximately 45° relative to the image plane of acamera/image detector of a central camera pod is equally applicable tothe calibration of other types of camera based patient monitoringsystem. Thus for example rather than identifying the locations ofcameras in a stereoscopic camera based monitoring system, the abovedescribed approach could equally be applied to determining the relativelocations of cameras and a treatment room isocenter in a time of flightbased monitoring system or alternatively a monitoring system based uponimaging the projection of structured light onto a surface beingmonitored.

Although in the above, the calibration of a monitoring system has beendescribed wherein a calibration cube 70 is positioned at a treatmentroom isocenter highlighted by the intersection of three planes of laserlight, it will be appreciated that the above described method couldequally be applied to other methods of identifying the relativelocations of a cameras in a patient monitoring system and the isocenter.

Thus for example, rather than relying upon the identification of thetreatment room isocenter being identified by the intersection of aplanes of laser light an approach such as the approach described inVision RT's earlier US Patent Application, US 2016-129283 could beutilized.

In such an approach, initially a calibration phantom in the form of acalibration cube containing irradiation targets, such as one or moresmall metallic balls or other metal targets made of steel, titanium ortungsten or the like, is positioned with the phantom's center at anestimated location for the isocenter of a radio therapy treatmentapparatus with the front surface of the calibration cube in plan-viewangled at approximately 45° relative to the image plane of a cameraforming part of a monitoring system. The calibration phantom is thenirradiated using the radio therapy treatment apparatus. The relativelocation of the center of the calibration phantom and the isocenter ofthe radio therapy treatment is then determined by analyzing radiographicimages of the irradiation of the calibration phantom containing theirradiation targets.

In some embodiments, the calibration phantom can then be repositionedby, for example, sending instructions to a moveable couch on which thecalibration phantom is mounted so as to apply an offset corresponding tothe determined relative location of the center of the calibrationphantom and the isocenter of the radio therapy treatment apparatus tothe calibration phantom. The relative location of the cameras of themonitoring system and the treatment room isocenter can then bedetermined by capturing images of the repositioned calibration cubepositioned so as have its center located at the treatment roomisocenter.

Alternatively, as is proposed in US 2016-129283, the relative locationsof the cameras and the treatment room isocenter could be determinedwithout physically relocating the calibration cube. More specifically,the calibration cube could be positioned in the manner described aboveat an estimated location for the isocenter of a radio therapy treatmentapparatus. Images of the calibration cube with the front surface of thecalibration cube in plan-view angled at approximately 45° relative tothe image plane of a camera forming part of a monitoring system couldthen be obtained and a 3D computer model of the surface of the cubecould then be generated. The calibration cube could also be irradiatedwithout the cube being repositioned and radiographic images of theirradiated cube and in particular irradiation targets within the cubecould be obtained and processed to determine the relative location ofthe cube and the treatment room isocenter. The location of the treatmentroom isocenter relative to the positions of the cameras of themonitoring system could then be determined based on any offsetdetermined by analyzing the radiographic images and the representationof the cube in camera space generated by processing images captured bythe monitoring apparatus.

It will be appreciated that adopting either approach described aboveavoids errors arising due to any inaccuracies with which the laserhighlighting system identifies a treatment room isocenter as in eitherapproach the treatment room isocenter is determined through the analysisof the images of the irradiation of targets contained within thecalibration cube 70. In such embodiments, it would be possible to omitthe presence of a laser highlighting system. However, preferably thelaser highlighting system would not be omitted as a laser highlightingsystem, together with markings 86-90 on the surface of a calibrationcube 70 facilitates an initial positioning of a calibration cube in thecorrect orientation and with its center very close to, if not perfectlyaligned with, a treatment room isocenter.

Although, in the example illustrated in FIGS. 4 and 6, a group of camerapods 72-76 are shown where all of the camera pods are located on thesame side of a plane passing through the isocenter of the treatmentroom, it will also be understood that the camera pods 72-76 might bearranged in a different configuration.

In particular it will be appreciated that the above described approachto calibration of a monitoring system would equally apply to amonitoring system where the secondary camera pods 74,76 are aligned witha plane passing through the treatment room isocenter parallel orsubstantially parallel to the image plane of the cameras/image capturedevices of a central camera pod 72 such as is illustrated in FIG. 8. Insuch a configuration, as is shown in FIG. 8, each of the surfacesclosest to each of the cameras 72-76 is at approximately 45° inplan-view angled relative to the image plane of the camera imaging thatsurface. Thus in such a configuration, the relative obliqueness at whichthe camera's view the calibration phantom 70 is minimized while stillenabling the monitoring system to view all four sides and the topsurface of the calibration phantom 70. In the case of the calibration ofa camera system comprising three camera pods 72-76 where two of thecamera pods 74,76 are arranged substantially symmetrically either sideof a central camera pod 72, angling the surface of the cube in plan-viewso as to be at approximately 45° relative to the image plane of thecameras/image capture devices of a central camera pod 72 is preferablebecause the symmetry of the arrangement of the camera pods 74,76 and thesymmetry of the calibration cube 70 causes the secondary camera pods74,76 to capture similar images of the calibration cube 70 and hence anyerrors arising based on the images captured by the secondary camera pods74,76 when determining the location of the calibration cube shouldcancel each other out.

It will also be appreciated that in other embodiments a monitoringsystem involving a single camera pod (e.g. the central camera pod 72alone) could be calibrated utilizing the approach described aboveorientating a calibration cube 70 so that the front surface of the cube(i.e. the surface closest to the image plane of the camera/imagedetectors with the camera pod 72 monitoring the cube in plan-view wasinclined relative to the image plane of the camera(s)) such as isillustrated in FIG. 9.

It will also be appreciated that in other systems, the central camerapod 72 might be omitted and instead a monitoring system including pairof camera pods 74,76 might be calibrated using the approach describedabove such as is illustrated in FIG. 10.

Although in the above described embodiments the alignment of thecalibration cube has been described as being such that the surfaces ofthe cube in plan-view are at approximately 45° relative to the imageplane of the cameras/image capture devices of a camera pod 72, it willbe appreciated that the alignment of the cube need not be exactly at 45°in order to obtain the benefits of the current invention and that somedeviation from 45° would be permissible.

1. A method of determining the relative location of an image capturedevice of a monitoring system for monitoring the positioning of apatient during radiation treatment and an isocenter of a treatment roomtowards which a treatment apparatus is arranged to direct radiation, themethod comprising: locating a calibration phantom in a treatment room,wherein in plan-view at least one surface of the calibration phantomclosest to at least one image capture device is inclined approximately45° relative to the camera plane of that image capture device; obtainingan image of the calibration phantom using the image capture device;processing an obtained image to identify the relative location of thecenter of the calibration phantom and the camera plane of the imagecapture device; and utilizing the identified location of the center ofthe calibration phantom and the camera plane of the image capture deviceto determine the relative location of the camera plane of the imagecapture device and the isocenter of a treatment room towards which thetreatment apparatus is arranged to direct radiation.
 2. A method inaccordance with claim 1 wherein the calibration phantom is configured tobe located with its center approximately at the isocenter of thetreatment room using a laser lighting system to highlight the isocenterof the treatment room; and, wherein in plan-view the calibration phantomis positioned such that the surface of the calibration phantom closestto the at least one image capture device is inclined approximately 45°relative to the camera plane of that image capture device by aligninglaser light, used to highlight the isocenter of the treatment room, withmarkings provided on the exterior of the calibration phantom.
 3. Amethod in accordance with claim 2 wherein the calibration phantomcomprises a cube and the markings provided on the exterior of thecalibration phantom comprise markings selected from a group comprisingmarkings extending along the edges of the cube; markings bisecting thecube; and a cross extending between diagonally opposite corners of thecube.
 4. A method in accordance with claim 1 wherein the calibrationphantom contains one or more irradiation targets, the method furthercomprising: obtaining a radiographic image of the calibration phantomirradiated by the treatment apparatus; analyzing the obtainedradiographic image of the calibration phantom to determine the relativelocation of the treatment room isocenter and the center of thecalibration phantom.
 5. A method in accordance with claim 4, furthercomprising repositioning the calibration phantom so that the center ofthe calibration phantom is located at the isocenter of the treatmentroom.
 6. A method in accordance with claim 1 wherein the monitoringsystem comprises a plurality of image capture devices.
 7. A method inaccordance with claim 1 wherein the monitoring system further comprisesa projector operable to project light onto the surface of an objectlocated in the vicinity of the isocenter of the treatment room.
 8. Amethod in accordance with claim 7 wherein the projector is operable toproject structured light in the form of a grid pattern or a line oflaser light onto the surface of an object located in the vicinity of theisocenter of the treatment room, wherein processing an obtained image togenerate a model of the imaged surface of the calibration phantomcomprises processing an obtained image of the calibration phantom ontowhich structured light has been projected and generating a model of theimaged surface of the calibration phantom on the basis of the distortionof a pattern of structured light appearing in the image.
 9. A method inaccordance with claim 7 wherein the projector is operable to project aspeckled pattern of light onto the surface of an object located in thevicinity of the isocenter of the treatment room, wherein the monitoringsystem comprises a stereoscopic camera system and processing an obtainedimage to generate a model of the imaged surface of the calibrationphantom comprises processing obtained stereoscopic images of thecalibration phantom onto which a speckled pattern of light has beenprojected and generating a model of the imaged surface of thecalibration phantom on the basis of identification of correspondingportions of an imaged object in stereoscopic images obtained by thestereoscopic camera.
 10. A method in accordance with claim 1 wherein themonitoring system comprises one or more camera pods each containing oneor more image capture devices wherein locating a calibration phantomwith its center located approximately at the isocenter of a treatmentroom, the surface of the calibration phantom closest to an image capturedevice being inclined approximately 45° relative to the camera plane ofan image capture device comprises locating a calibration phantom withits center located approximately at the isocenter of the treatment room,wherein the surface of the calibration phantom closest to an imagecapture device of one of the camera pods is inclined approximately 45°relative to the camera plane of the image capture device on that camerapod.
 11. A method in accordance with claim 10 wherein the one or morecamera pods comprise a plurality of image capture devices and the camerapods are all located on the same side of a treatment room.
 12. A methodin accordance with claim 10 wherein the one or more camera pods arearranged in a symmetrical pattern within the treatment room.
 13. Amethod in accordance with claim 11 wherein the monitoring systemcomprises three camera pods each containing one or more image capturedevices wherein locating a calibration phantom with its center locatedapproximately at the isocenter of a treatment room, in plan-view thesurface of the calibration phantom closest to an image capture devicebeing inclined approximately 45° relative to the camera plane of animage capture device comprises locating a calibration phantom with itscenter located approximately at the isocenter of a treatment room,wherein in plan-view the surface of the calibration phantom closest toan image capture device of a centrally located camera pod, flanked bytwo other camera pods is inclined approximately 45° relative to thecamera plane of the image capture device on the centrally located camerapod.
 14. A method in accordance with claim 10 wherein the camera podsare suspended from the ceiling of the treatment room.
 15. A monitoringsystem for monitoring the positioning of a patient during radiationtreatment and comprising a computer and an image capture device, whereinat least one image capture device is configured to obtain an image of acalibration phantom and wherein the computer is configured to:determining a relative location of the image capture device and anisocenter of a treatment room towards which a treatment apparatus isarranged to direct radiation based on obtaining an image of thecalibration phantom using the image capture device, wherein in plan-viewat least one surface of the calibration phantom closest to at least oneimage capture device is inclined approximately 45° relative to thecamera plane of that image capture device; processing an obtained imageof the calibration phantom to identify the relative location of thecenter of the calibration phantom and the camera plane of the imagecapture device; and utilizing the identified location of the center ofthe calibration phantom and the camera plane of the image capture deviceto determine the relative location of the camera plane of the imagecapture device and the isocenter of the treatment room towards which thetreatment apparatus is arranged to direct radiation.
 16. A monitoringsystem in accordance with claim 15 further comprising a laser lightingsystem to highlight the isocenter of a treatment room and wherein themonitoring system is configured to the laser lighting system isconfigured to aligning laser light to highlight the isocenter of atreatment room with markings provided on the exterior of the calibrationphantom.
 17. A monitoring system in accordance with claim 1 wherein thecalibration phantom contains one or more irradiation targets.
 18. Amonitoring system in accordance with claim 1 further comprising aprojector configured to project light onto the surface of an objectlocated in the vicinity of the treatment room isocenter.
 19. A method inaccordance with claim 2 wherein the monitoring system comprises aplurality of image capture devices.
 20. A method in accordance withclaim 3 wherein the monitoring system comprises a plurality of imagecapture devices.